Bioprinting has a bright, but blurry, future

Bioprinting is layer-by-layer manufacturing using living cells. Small groups of cells are “printed” in precise patterns that can be built up in layers by printing one layer on top of another. With the right materials, these layers can form complex 3-dimensional structures, designed using clinical images from MRI and CT scans, that approximate living tissue. There are grande claims made by those who’s research involves bioprinting, such as one day being able to print functional organs for transplant, but its real future may not lie along those lines.

This technology started with a modified inkjet printer, the same as used for printing from any home computer. Inkjet bioprinters are still used, though modifications are made for more precise control of positioning and volume of “ink” dispensed. Christian Mandrycky, et al., have written a review in Biotechnology Advances that covers the current state of bioprinting for tissue engineering. They start with the techniques used to print cells, like inkjet printers, and discuss the materials used as bioinks and the applications seen so far.

“While no one-size-fits-all approach to bioprinting has emerged, it remains an on-demand, versatile fabrication technique that may address the growing organ shortage as well as provide a high-throughput method for cell patterning at the micrometer scale for broad biomedical engineering applications.” — Christian Mandrycky, et al.1

Four techniques used for bioprinting

What is bioink?

Bioprinters work by ejecting bioink from a printer nozzle. Bioink is usually living cells dispersed in some type of gel that will hold its structure after being printed. Most of these gels are natural polymers like collagen and alginate. Decellularized extracellular matrix is beginning to be used as bioink, as well.

How is bioink printed?

Inkjet printing is still the easiest technology to use. The equipment is inexpensive relative to other techniques, the printing speed is fast, and most of the cells survive the process. Inkjet printers push out ink by deforming the printer jets, generating droplets that are deposited on a surface. The main limitation of inkjet technology is that it cannot make droplets from the high density solutions of cells that are usually required for building tissue.

Commercially available bioprinters currently use extrusion technology. Extrusion can use the same type of positioning hardware as inkjet printers for creating patterns, but uses air pressure or plungers to push the bioinks out of the printer heads. There are no limitations to the inks that can be used with this method. The limitations are a higher amount of mechanical stress put on cells and the slow speed of printing. Printing speed is important because once the pattern has finished printing, the cells still need to be moved to an incubator to stay alive.

Laser printers are also used for bioprinting. The gel containing the cells is attached to a strip of metal, and a laser heats the opposite side. The heat creates a bubble in the gel that pushes a droplet of cells onto a surface. This method puts no mechanical stress on the cells, but it is very expensive. The cost is high enough that few laser bioprinters are used. The process has complex parameters that are still being investigated to optimize speed and resolution of the patterns it can create.

New technique

Similar to how patterns are created on semicondcutors for computer chips, stereolithography uses light to create the patterns rather than a printer head with bioink. Cells are dispersed in a layer of light-curable gel, one that hardens when exposed to certain wavelengths of light. A mask in the shape of the desired pattern hides portions of the gel from light exposure. After exposing the mask, a new layer of gel is put down and a second mask is used to create the second layer of the pattern. When all layers are hardened with light, the unexposed areas are washed away, leaving only the three-dimensional pattern. This method is new to bioprinting research, but benefits from workflows created for other applications.

How bioprinting is being applied

Successful applications

Current applications of bioprinting are far away from the promised organ replacements, but there are at least two areas where bioprinting has had success: individual blood vessels and drug screening systems.

Creating networks of blood vessels within dense tissue is an unresolved challenge for bioprinting, but small conduits that can be used as individual blood vessels have been created. To create a blood vessel, two different “inks” are printed at the same time. One bioink has the cells and gel that create the vessel walls, and the second ink is a sacrificial material that provides structure during the printing process, but can be washed away when the new blood vessel has enough strength on its own.2 This process works for simple tubes, but so far it is too difficult to completely wash out entire networks of tubes small enough to replicate the capillaries that feed tissue.

Drug screening systems are the real and present promise of bioprinting systems. Bioprinting cannot replicate functional organs, but it can replicate small sections of tissue that can test how different cells react in the presence of new drugs. Demonstrated with printed liver tissue, multiple cell types can be printed in the same space, even creating a gradient of cell types, that are able to demonstrate how cells metabolize drugs and reveal possible side effects. Bioprinting allows for testing of drugs in engineered living tissues that provide more realistic data than using two dimensional cell cultures. In addition, multiple trials can be performed at lower cost, and in a faster time frame, than when using animal testing.

Still working

Bioprinting has not had clear success with bone and cartilage tissue. While three dimensional printing of nondegradable polymers has been used to create scaffolds for bone tissue engineering, printing the bone tissue itself has not been successful. There have been demonstrations of making anatomical shapes, like an ear, by printing cartilage cells and nondegradable polymers at the same time, but a lack of functional blood vessel networks will keep this technique from clinical use anytime soon. Experiments with nerve tissues have mainly been simple demonstrations of cell viability using inkjet and extrusion systems.

A bright, but blurry, future for bioprinting

Bioprinting has a sure future in assembling viable blood vessels for transplant. Drug screening will probably be bioprinting’s most important role to play in the near future. However, in spite of the theoretical promise of transplantable, on-demand organs that can be made available whenever we get sick, this accomplishment is no where in sight with the current technology. It seems more likely that decellularized organs will find success before bioprinting makes anyone a new liver. Those looking for printable organs seem to have blurry vision when looking at this promising, but limited, technology.

Sacrificial materials allow bioprinters to create the hollow space needed for tubes and conduits without the walls and top surface caving in on themselves. After washing, a hollow space will remain where ever the sacrificial material is printed.